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SHARON SHANKS | The Cosmos Getting the phases right makes me a little moonstruck



Published: Sun, June 3, 2001 @ 12:00 a.m.



Did you ever say something and wonder why you ever let those words out of your mouth? I'm ashamed to say that I did -- at a public planetarium program, no less -- and I'm afraid that even Ann Landers' famed 40 lashes with a wet noodle won't atone in this case.

It all started with moon phases. I hate moon phases. Of all the concepts that schoolchildren are expected to master, the phases of the moon have to be the absolute worst, especially for people (like me) who have trouble conceptualizing in three dimensions.

The subject of moon phases has to be at the top of the list of things about space and astronomy that pupils can misunderstand. I can explain the whole idea of a synchronous orbit with words, but I always feel that the idea just isn't getting through. It's much easier to do as a demonstration.

Starting out: Let's start at the beginning. We have one natural satellite, which makes Earth the exception among the terrestrial planets. Mercury and Venus don't have moons and Mars has two, but they really don't count -- Phobos and Deimos are probably large asteroids from the asteroid belt that were nudged out of orbit by a collision or by Jupiter's gravity and were captured in Mars' gravitational field.

The most likely explanation for our moon is that we came by it by accident. Something the size of Mars collided with Earth very early in our planet's history and knocked off a massive amount of newly-formed crust. The crustal material and remains of the giant impactor were ejected into orbit around Earth and, thanks to rotational motion and Earth's gravity, formed the moon.

This theory explains why the composition of lunar rocks brought back by the Apollo astronauts were so eerily similar to Earth rocks.

From the very beginning, then, there has been a strong gravitational attraction between Earth and the moon. Anyone who lives near a coast knows the most obvious result: the tides. The moon's gravity pulls on the entire planet, and because water is fluid it responds to a greater degree. (Earth's crust also moves, but just slightly.) If we could see the effects of the moon's gravity from a point in space, we would notice two bulges: one in the direction of the moon and one opposite.

Slowing down: Another result of the moon's presence is a gradual slowing down of Earth's rotation. The bulges created by the moon's gravity aren't directly in line from the center of the Earth to the center of the moon -- they are carried by Earth's spin slightly ahead of the point that's directly below the moon. Like a whiplash ride at a carnival, this creates torque and, as a result, rotational energy from the Earth is transferred to the moon.

Long before the first life appeared in Earth, the days were much shorter and the moon, much closer. After several millennia of transferring energy, the Earth's rotation has slowed to one spin in 24 hours and is continuing to slow at a rate of 1.5 milliseconds (.0015 seconds) per century. The moon, accelerated by the Earth, has been raised to a higher orbit and is pulling away from our planet at a rate of 3.8 centimeters (1.5 inches) per year.

The stage is set to discuss the dreaded subject of the & quot;dark side of the moon. & quot; There is no such thing, of course. The moon, just like the Earth, is half lighted by the sun all the time. But we're seeing the moon from Earth, and unless you're among the lucky ones who can visualize these things, what we see doesn't necessarily jibe with what's taking place.

The gravitational interaction between the Earth and moon has locked the moon into a synchronous orbit: The same side of the moon is always facing the Earth. The Earth's gravitational pull on the moon also created a bulge, but in the case of the waterless moon, it is crust that has been pulled into place. The moon's bulge aligned itself with Earth and it fell into a stable orbit.

What we can see: We see the result of this gravitational interplay every time we look at the moon: We see the same & quot;face & quot; all the time. The moon does rotate, but its rotation matches its orbit. Let's draw a picture for this one. Draw a circle to represent Earth, and draw another smaller circle at the 12 o'clock position for the moon. Mark an & quot;x & quot; on the moon circle for the side that's facing the Earth. Now draw three more circles, at 9, 6 and 3 o'clock. Keep each & quot;x & quot; facing the Earth.

The only way those & quot;x & quot; marks could face the Earth at all times is if the moon rotated. If it didn't, the & quot;x & quot; marks would face directly toward Earth only once during the month.

The phases of the moon are a result of our perspective on Earth. We can see only the part of the moon that is being lit by the sun. On your diagram, make a circle for the sun far beyond the Earth and moon at the 12 o'clock position. In this alignment, we can't see any of the lit side of the moon -- we call this phase new moon. At 9 o'clock, half of the moon is still lighted, but we can see only half of the half-lighted part that is facing us, and we call this first quarter.

At 6 o'clock, we can see the entire lit face of the moon, and we have full moon. At 3 o'clock, again half of the moon is still lighted but we can see only half of the half, and we call this last quarter.

So what did I say that was so heinous that I'm still having nightmares about it? I told my audience that we couldn't see the far side of the moon because it's facing the Earth at new moon. Aargh! I know better -- that's one of those misconceptions that just won't go away, and I certainly didn't help it die. All I can say to my audience is bring wet noodles with you the next time you come to the planetarium.




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